The present invention relates to throttle control mechanisms of carburetors for internal combustion engines, and more particularly to such a mechanism incorporating a detent mechanism for yieldably holding and positioning the throttle valve in one or more of a predetermined plurality of operational positions.
Manually operated throttle valve control levers are typically provided on small carburetors designed for use with low displacement gasoline fueled engines, such as used on chain saws, weed whips, leaf blowers, and other small lawn, garden and forestry portable appliances. Although the throttle valve is typically operator manipulated for angular travel throughout an operable range from closed to wide-open, a throttle control detent mechanism is customarily provided for yieldably holding the throttle valve in a selected one of two or three predetermined operating positions, e.g., namely wide-open throttle (W.O.T.), idle and fully closed. On larger lawn and garden appliances the detent mechanism may be built into the throttle control linkage parts, such as a control knob protruding through a control panel slot having notches along the travel path of the control knob arm. However in very small lawn and garden appliances, such as weed whips and leaf blowers, the engines are typically of small size and of low displacement, and therefore typically are provided with a cubic-type diaphragm carburetor that may only be between one and two inches square in outside dimensions. The throttle control linkage may only consist of a single lever fixed at one end to the throttle shaft and protruding to a finger-grip free end located in an operator-accessible zone adjacent to the carburetor mounting location on the engine.
Heretofore the typical detent mechanism utilized in such small carburetor throttle control mechanisms consists of a conventional ball and spring detent. This type of throttle detent mechanism requires that a blind bore be provided in the carburetor body for receiving the compression coil spring as well as the hardened steel ball that seats against the free-end of the spring. The spring-biased ball rides against the throttle shaft circumference and is forced into whichever one of three throttle shaft pockets comes into registry with the ball during throttle shaft rotation.
Due to the minuscule dimensions of these detent mechanism parts involved in such tiny carburetors, and particularly the very small diametrical dimension of the throttle shaft, it is difficult in the first instance to machine the detent pockets on the throttle shaft, and even more difficult if not economically impossible to angularly locate the pockets to the sufficiently close manufacturing circumferential dimensional tolerances that would be required in order to accurately establish within close angular tolerances the predetermined positive position stops for the throttle valve at W.O.T., idle and closed valve settings.
In addition, there are the usual manufacturing and assembly costs involved in providing the spring and ball type throttle control detent mechanism, and these costs are particularly aggravated when producing very small cubic carburetors of the aforementioned type.
Accordingly, among the objects of the present invention are to provide an improved carburetor throttle control detent mechanism that eliminates the need for the aforementioned ball and spring type detent mechanism and yet is also built into the carburetor assembly and hence does not require any cooperative construction either on the engine or the appliance on which the engine is installed, that achieves reduced costs of manufacture and assembly and yet is capable of controlling the throttle valve clocking operation in very small and precise increments, and that allows a choice of a plurality of predetermined positive detent stop positions for the throttle valve throughout the range of throttle valve operation from W.O.T. to fully closed.
Another object of the invention is to provide an improved carburetor throttle control detent mechanism of the aforementioned character which, when employed on a carburetor having a choke valve shaft, is capable of utilizing the choke control shaft as one of the cooperative detent stops in the detent control mechanism.
A further object of the present invention is to provide an improved carburetor throttle control detent mechanism of the aforementioned character in which the throttle control lever and the detent cam member of the mechanism are combinable into one unitary piece part in order to further reduce overall cost of manufacture and assembly of the carburetor and associated throttle control mechanism.
In general, and by way of summary description and not by way of limitation, the invention fulfills one or more of the foregoing objects by providing a carburetor having a body with an air-fuel mixture passageway and a rotatable throttle valve in said passageway mounted for rotation on and with a throttle shaft that is journaled for rotation on a rotational axis in said body. The throttle shaft has a free end protruding exteriorly from an exterior side surface of the body. Typically the throttle shaft has a given diameter of relatively small dimension. A throttle lever detent arm is mounted on the throttle shaft free end for rotation therewith in an angular travel path about the rotational axis and adjacent the body exterior side surface. First detent means are provided on the body side surface located in fixed position adjacent the travel path of the detent arm and spaced radially away from the rotational axis a predetermined distance greater than the diameter of the throttle shaft by a multiple of the shaft diameter dimension, e.g., a distance about three times the shaft diametrical dimension. Second detent means are provided on the detent arm that likewise are generally spaced such predetermined distance radially away from the rotational axis. The detent means are constructed and oriented so as to be releasibly engageable with one another for thereby yieldable holding the detent arm and hence the throttle shaft and associated throttle valve in any one of a plurality of selected angular settings.
One of the primary features of the carburetor throttle control detent mechanism of the invention is providing engagement of the first and second detent means, regardless of their structural form, in an arc of mutual engagement along a detent arm cam travel path having a radial dimension, centered on the throttle shaft rotational axis, that is a multiple of the small diameter dimension of the throttle shaft, such as a multiple of three times the shaft diametrical dimension. This large radius of the travel path of the arcuate cam control surface thus allows the tolerance limits of the radial variations that are spaced circumferentially apart along the cam surface to be manufactured to the same manufacturing tolerance specifications that are otherwise normally employed in machining a detent ball seating groove in the throttle shaft when providing the prior art ball/spring detent mechanism.
Yet in so doing, and without tightening up prior manufacturing tolerance specifications, the angular tolerance variation on the set positions of the throttle valve blade as controlled by the detent cam surface is now approximately three times more precise, i.e. the tolerance limits for the detent controlled predetermined angular positions of the throttle valve are now rendered three times tighter than otherwise would be possible when utilizing the prior control detent pockets provided in the surface of the throttle shaft. Hence manufacturing tolerances do not need to be tightened up in order to achieve a three-fold improvement in operational tolerances of the carburetor throttle control detent mechanism. The invention thus provides a low cost throttle control detent mechanism that enables fine increment, positive stops at predetermined valve blade settings, such as the W.O.T. (wide-opened throttle), idle and closed valve positions.
In one embodiment the throttle lever detent arm comprises a planer segment of a circle with an arcuate cam surface having radial variations therein spaced circumferentially therealong and forming the first detent means, the second detent means comprises a cam follower means fixedly supported on the body side surface and yieldably engaging and tracking on the segment cam surface and registerable with the radial variations therealong for releasibly holding the detent arm segment in any one of the plurality of settings as determined by location of the radial surface variations circumferentially along the segment cam surface. The segment cam surface comprises an arcuate peripheral free edge and the segment is constructed to have void means located adjacent the segment free edge to thereby add resilience to the free edge engagement with the cam follower means tracking therealong and also to thereby reduce the mass of the detent arm segment. Preferably the radial variations of the cam surface comprise indentations spaced circumferentially therealong in predetermined locations corresponding to the plurality of selected angular settings of said throttle valve. The cam follower means preferably comprises a spring biasing pin cantilever mounted in the body side surface and yieldably registerable with the indentations when the same are individually brought into angular alignment with the pin in response to rotation of the throttle shaft.
Preferably one of the indentations is designed to correspond to the closed position of the throttle valve and comprises an inclined surface oriented such that engagement with the cam follower pin develops a torque on the detent arm segment in a direction tending to further close said throttle valve to thereby maintain a closing bias on the throttle valve during such engagement.
In another embodiment one of the radial edge variations of the detent arm segment peripheral edge comprises a concavity, and the carburetor has a rotatable choke shaft with a choke valve mounted on said choke shaft for rotation therewith. The choke shaft has a free end protruding from the carburetor body side surface adjacent the travel path of the segment free edge and oriented to function as one of the second detent means by yieldable registry engagement of the segment edge concavity therewith.
In yet another embodiment the carburetor also has a throttle lever constructed for manual manipulation for swinging through an operational range corresponding to the angular operational range of said throttle valve. The lever is operable to impart operator torque forces on the throttle shaft for rotating the same, and the throttle lever detent arm is integrated with the throttle lever and fixed thereto for co-rotation therewith. The throttle lever and throttle lever detent arm may be integrated into a unitary part so that they are co-planar with one another.
Preferably the lever portion of the unitary part has additional void means radially outwardly of the segment free edge for reducing the overall mass of the part.
In still another embodiment the first detent means on the carburetor comprises at least one pocket concavity formed in the body side surface and the second detent means comprises a lateral projection on the throttle lever detent arm oriented to ride on the body side surface and operably snap into the pocket concavity upon registry therewith by spring bias developed in the throttle lever detent arm. This embodiment provides the additional advantage of using the lateral projection as a replacement for the axially biasing spring typically found on current throttle controls.
In a reversal embodiment the radial variations along the arcuate cam surface of the detent arm comprise radial protrusions spaced circumferentially therealong corresponding to the selected angular settings of the throttle valve. The cam follower means comprises a semi-resilient paddle member having a shallow xe2x80x9cWxe2x80x9d configuration in radial cross section adapted to slidably bear on the cam surface and to be cammed over and then individually registered with the radial protrusions. The cam follower has a stem portion received in a mounting opening in the side surface of the carburetor body for cantilever support therefrom of the cam follower means.
In a still further embodiment the throttle lever detent arm comprises a pair of angularly spaced apart, radially extending support legs joined at one end to a hub mounted on the throttle shaft free end. The radially outermost distal ends of these support legs carry an arcuate cam track member having the arcuate cam surface thereon and the radial variations formed therein to provide such first detent means.